The other two answers suggest that either this is irrelevant, or that making the disclosure might imply that being close friends is somehow a bad thing.
It's both relevant, and a fairly normal potential conflict of interest which should be disclosed.
Any time there is a possibility that a relationship will unfairly or inappropriately affect an outcome, a disclosure is appropriate.
There are two reasons to make disclosures of potential conflicts of interest. The first is to ensure that there is an objective decision making process, even when there may be some bias. Interviewer is a friend? Sure, but the interview questions, process, time, etc. was reviewed by someone who was neutral and found to be appropriate and objective. The second is to foreclose any possibility of impropriety. Someone discovers later than the interviewer was a friend? Sure, but Human Resources or the upline managers were well aware of the fact.
I recently bought a house which came with appliances I didn't need since I moved with my own. I overheard a co-worker mention he was looking to replace some of his appliances. I offered them to him. I believe a fair-market value for what I gave him was somewhere between $500 and $1,000. That was more than sufficient to influence him to potentially be more favorable to me, so I informed my manager of the gift. I also informed my co-worker that I'd reported the gift to my boss. Everyone was happy and he loves the appliances I gave him, and I love not having to go through the grief of selling things which could have been a major hassle.
We don't disclose conflicts because we think friendships are bad, we disclose them because it prevents the conflict itself from creating problems.